This invention is directed toward producing inorganic coatings with reduced light scattering or haze. More particularly, it is directed to producing energy-conserving windows with clearer transmission of light and less scattering of light.
It is well known in the art to produce coatings on window glass that reduce the flow of heat through the window. These "low-emissivity" coatings reduce the emission of infrared heat rays from the coated surface of the window. The coatings contain either a metal such as silver, or a semiconductor such as fluorine-doped tin oxide. The metal coatings ("soft coats") have good optical properties and show little haze, but they are not durable to abrasion, or to exposure to air or moisture. The semiconductor coatings ("hard coats") are mechanically and chemically extremely durable, but tend to show more haze.
Haze is measured by instruments which detect light which is transmitted through the coated sheet, but is deflected by more than a few degrees from the original direction of the light. Coated glass with haze values of more than about one percent may be considered objectionable for windows. Under special viewing conditions (bright light, dark background) haze may be seen on coated glass with haze values of even less than one percent.
Among the mechanisms that produce haze in a coating, are irregularities at either of the surfaces of the coating, or voids within the coating. Roughness on the outer surface of the coating has sometimes been reduced by polishing, but this adds a difficult step to the manufacturing process. Irregularities at the glass-tin oxide interface are sometimes introduced by reactions producing salt crystals from sodium in the glass combining with elements such as chlorine contained within the chemical precursors used to form the tin oxide. Such salt crystals can be avoided by first depositing on the glass a layer such as silica or alumina which inhibits diffusion of sodium.
A great deal of effort has gone into reducing the light scattering or haze of commercially-produced tin oxide coatings on glass, and products are now produced with haze values below one percent. However, it is considered important to reduce haze values still further.